ABSTRACT Age-related inflammation (so-called inflammaging) is associated with many deleterious processes in the elderly, including Alzheimer's disease, cardiovascular disease, and general frailty. Declining immune function is also well described in the elderly, and leads to increased risk and severity of infection, poorer control of cancer, and impaired responses to vaccination. While there is clearly cell-intrinsic immune dysfunction with age, our and others data have revealed that cell extrinsic mechanisms also play significant roles. Our long-term goal is to understand the mechanisms underlying the dysregulation of immune responses in aging and develop therapeutic strategies to enhance protective immune responses in the elderly. Our new data show that the key anti-inflammatory cytokine IL-10 is systemically increased in aged mice, and blockade of IL-10 significantly reverses the age-driven impairment in vaccine responsiveness. Investigating the cellular sources of increased IL-10 through single-cell RNA sequencing and flow cytometric analysis, we found that a sub-population of memory CD4+ T cells, largely comprised of T follicular helper (Tfh) cells, which we are referring to as Tfh10 cells, accumulate with age and are required for systemically increased IL-10 in aged mice. These Tfh10 cells are also significantly increased in aged humans. Mechanistically, we show that in mice, IL-6 and IL-21 are non-redundant and essential for Tfh10 cell accumulation. Furthermore, our data suggest that IL- 6 controls the development and not the maintenance of Tfh10 cells, whereas IL-21 is required to suppress expression of the pro-apoptotic molecule Bim in Tfh10 cells, suggesting that IL-21 promotes their survival. Moreover, our preliminary data suggest a novel control of IL-10 production in these Tfh10 cells, as we found that Bcl-6, the canonical Tfh transcription factor, is downregulated in aged Tfh10 cells, and genetic loss of Bcl-6 enhanced IL-10 expression in FoxP3- CD4+ T cells. Finally, our ATACseq data show several regions of open chromatin in the IL-10 locus in Tfh10 cells that are: conserved between human and mouse; are not present in nave T cells; and possess validated binding sites for repressive (Bcl6) and activating transcription factors (STAT3 and AP1). These compelling preliminary data drive our overall hypothesis that IL-6 and IL-21 promote the development and survival of IL-10-producing Tfh cells, whose IL-10 signaling to B cells is critical to limit durable, protective vaccine responses. We will address these hypotheses through 3 Specific Aims. We will (1) identify the cellular loci of action and function of IL-10 on vaccine-driven protective immunity in aging; (2) determine the cellular mechanism(s) that control the accumulation of Tfh10 cells with age, and (3) determine the molecular mechanism(s) that control IL-10 expression in Tfh10 cells.